This disclosure relates generally to electrical interconnection systems and more specifically to improved signal integrity in interconnection systems, particularly in high speed electrical connectors.
Electrical connectors are used in many electronic systems. It is generally easier and more cost effective to manufacture a system on several printed circuit boards (“PCBs”) that are connected to one another by electrical connectors than to manufacture a system as a single assembly. A traditional arrangement for interconnecting several PCBs is to have one PCB serve as a backplane. Other PCBs, which are called daughter boards or daughter cards, are then connected through the backplane by electrical connectors.
Electronic systems have generally become smaller, faster, and functionally more complex. These changes mean that the number of circuits in a given area of an electronic system, along with the frequencies at which the circuits operate, have increased significantly in recent years. Current systems pass more data between printed circuit boards and require electrical connectors that are electrically capable of handling more data at higher speeds than connectors of even a few years ago.
One of the difficulties in making a high density, high speed connector is that electrical conductors in the connector can be so close that there can be electrical interference between adjacent signal conductors. To reduce interference, and to otherwise provide desirable electrical properties, shield members are often placed between or around adjacent signal conductors. The shields prevent signals carried on one conductor from creating “crosstalk” on another conductor. The shield also impacts the impedance of each conductor, which can further contribute to desirable electrical properties. Shields can be in the form of grounded metal structures or may be in the form of electrically lossy material.
Other techniques may be used to control the performance of a connector. Transmitting signals differentially can also reduce crosstalk. Differential signals are carried on a pair of conducting paths, called a “differential pair.” The voltage difference between the conductive paths represents the signal. In general, a differential pair is designed with preferential coupling between the conducting paths of the pair. For example, the two conducting paths of a differential pair may be arranged to run closer to each other than to adjacent signal paths in the connector. No shielding is desired between the conducting paths of the pair, but shielding may be used between differential pairs. Electrical connectors can be designed for differential signals as well as for single-ended signals.
Differential connectors are generally regarded as “edge coupled” or “broadside coupled.” In both types of connectors the conductive members that carry signals are generally rectangular in cross section. Two opposing sides of the rectangle are wider than the other sides, forming the broad sides of the conductive member. When pairs of conductive members are positioned with broad sides of the members of the pair closer to each other than to adjacent conductive members, the connector is regarded as being broadside coupled. Conversely, if pairs of conductive members are positioned with the narrower edges joining the broad sides closer to each other than to adjacent conductive members, the connector is regarded as being edge coupled.
Maintaining signal integrity can be a particular challenge in the mating interface of the connector. At the mating interface, force must be generated to press conductive elements from the separable connectors together so that a reliable electrical connection is made between the two conductive elements. Frequently, this force is generated by spring characteristics of the mating contact portions in one of the connectors. For example, the mating contact portions of one connector may contain one or more members shaped as beams. As the connectors are pressed together, each beam is deflected by a mating contact, shaped as a post or pin, in the other connector. The spring force generated by the beam as it is deflected provides a contact force.
For mechanical reliability, contacts may have multiple beams. In some implementations, the beams are opposing, pressing on opposite sides of a mating contact portion of a conductive element from another connector. In some alternative implementations, the beams may be parallel, pressing on the same side of a mating contact portion.
Regardless of the specific contact structure, the need to generate mechanical force imposes requirements on the shape of the mating contact portions. For example, the mating contact portions must be large enough to generate sufficient force to make a reliable electrical connection. These mechanical requirements may preclude the use of shielding, or may dictate the use of conductive material in places that alters the impedance of the conductive elements in the vicinity of the mating interface. Because abrupt changes in impedance may alter the signal integrity of a signal conductor, mating contact portions are often accepted as being noisier portions of a connector.